1. Field of Invention
The present invention relates to an optical fiber transmitter used for long-distance subcarrier multiplexed (SCM) lightwave systems. More particularly, the invention relate to the transmitter using a chirped fiber grating to correct the bulk part of nonlinear distortion, and an electrical predistorter to correct the residual nonlinear distortions.
2. Description of Related Art
Chromatic dispersion in an optical fiber system causes severe degradation of transmission quality. This occurs because chromatic dispersion, fiber non-linearity and the laser frequency chirping (produced by directly modulating a semiconductor laser), can combine to generate nonlinear distortions. For an SCM lightwave system, the non-linear distortions are characterized by one of the following parameters: composite second order (CSO) distortions and composite triple beats (CTB) in analog AM-VSB cable television systems; and carrier-to-intermodulation (C/IM) in digital M-QAM and digital wireless systems. The nonlinear distortions caused by chromatic dispersion becomes more pronounced as the laser frequency chirping increases, the number of RF/microwave channels increases, and optical modulation index per channel (OMI/ch) increases. For example, let us consider a direct modulation system (i.e., using directly modulated laser diode) with 80 analog AM-VSB channels and OMI/ch=4%. If the laser frequency chirp is 6 GHz, then the fiber chromatic dispersion will produce CSOs of about xe2x88x9230 dBc (the requirement is less than about xe2x88x9263 dBc) after an optical fiber transmission distance of 60 km. Therefore, it is necessary to compensate chromatic dispersion so that a long-reach directly modulated SCM system is feasible.
In external modulation systems, there is no concern on laser frequency chirping because the laser is running CW. However, nonlinear distortions can be caused by optical fiber dispersions combining with various factors such as: stimulated Brillouin scattering [Optical Fiber Communication Conference ""97, postdeadline paper, PD23-1], self-phase modulation [IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 3, No. 5, (May, 1991), pp. 481-483], and cross-phase modulation [JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 17, No. 2, (February 1999), pp. 178-190]. In addition, there is the so-called carrier suppression effect caused by the combination of optical fiber dispersion and the large external phase modulation on the external modulator. Therefore, regardless of direct or external modulation systems, chromatic dispersion induced nonlinearities must be corrected by using either optical or electrical techniques.
Furthermore, when a 1.55 xcexcm-band optical fiber amplifier is used in the transmission path of a 1.3 xcexcm-band single mode fiber (SMF) having been already installed, large positive chromatic dispersion of these fibers in the 1.55 xcexcm-band becomes a serious problem. Therefore, a dispersion-compensating fiber having large negative chromatic dispersion in the 1.55 xcexcm wavelength band has been developed. Such a dispersion-compensating fiber (DCF) is disclosed in ELECTRONICS LETTERS, Vol. 30, No. 2, (Jan. 20, 1994), pp. 161-162. However, DCF has the following drawbacks: it could have high nonlinearities caused by high launched optical power (mainly because of its smaller fiber core diameter), it has a large insertion loss, it has a bulk volume, and its cost is high.
On the other hand, chirped fiber grating has been proposed as an alternative way of compensating chromatic dispersion. For example, as disclosed in Optical Fiber Communication Conference ""94, postdeadline paper-2, PD2-1 to PD2-4. Recently a linearly chirped fiber grating apodized (or tapered) to alleviate the harmful ripples on reflection and group delay characteristics has been studied, and has been proposed to be used in SCM systems. For example, as disclosed in JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 15, No. 2, (February 1997), pp. 179-187. A fiber grating is made of a photo-sensitive optical fiber, with a periodic refractive index change formed in the fiber core by using the ultraviolet rays. A diffraction grating can thus be obtained such that a specific wavelength can be reflected by the diffraction grating.
The refractive index change, that is, the period of the grating is designed to be gradually reduced from the incident side toward the right so that an optical signal of a longer wavelength is reflected at a position nearer the incident side, i.e., longer wavelength experiences a shorter delay. The chirped fiber grating operates so that the wavelength reflected by the aforementioned diffraction grating is shifted in the direction of the length of the fiber, that is, chirped. Since the chromatic dispersion in a conventional 1.3 xcexcm-zero-dispersion optical fiber causes a longer delay in longer wavelength, a chirped fiber grating can be designed to compensate its effect.
Chirped fiber gratings have been demonstrated as effective chromatic dispersion compensation devices to overcome the chromatic dispersion induced limitations over the long-haul transmission paths. They provide a simple and attractive optical fiber delay, which is polarization-insensitive, inherently fiber compatible, relatively easy to produce, passive and low loss. However fiber gratings are reflective and resonant devices. Uniform strength, chirped fiber gratings have sidelobes in the reflection spectra and nonlinear dispersion characteristics, which are undesirable for optical communications. These effects come from the interference of small reflections from the end of the grating. Fortunately, they can be significantly suppressed by apodizing the grating. An optimized apodization profile improves the performance of the compensation device in the system; however, there still remain slight pseudo-periodic ripples on the reflectivity and group delay characteristics. The period of these ripples depends in the length and the bandwidth of the grating, and has roughly a range of several tens to several hundreds of picometers. In addition, the imperfect fabrication process introduces stochastic variations in the group delay and reflectivity responses. The effects of these group delay and reflectivity ripples on the grating performance in high-speed optical communication systems are investigated in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 10, No. 10 (October 1998), pp. 1476-1478.
A chirped fiber grating used for chromatic dispersion compensation is usually designed for a fixed transmission distance unless the chirp amount of the fiber grating is tunable. However, there is usually an about 10% inaccuracy of chromatic dispersion in commercially available chirped fiber gratings. As a result, the extra or insufficient chromatic dispersion due to the employment of a chirped fiber grating can cause additional signal nonlinear distortions in SCM systems.
From the above discussions, we see that the use of a chirped fiber grating can indeed compensate the majority of transmission system chromatic dispersions, but it also causes some residual side effects which are damaging to the signal quality. These side effects are due to its non-ideal dispersion and reflection characteristics, and due to the errors in the amount of dispersions that can be offered by a commercial product. To overcome these problems, we propose using an electrical pre-distortion circuit before the optical laser diode or external modulator. This circuit is designed to generate a controllable amount of second- and third-order nonlinear distortions to compensate those generated from the residual effects aforementioned.
The invention provides an optical fiber transmitter for long-distance SCM systems. The optical fiber transmitter includes an electrical pre-distortion circuit, an optical modulator, which includes, for example, a laser diode or an external modulator, connected to the pre-distortion circuit, an optical signal directing unit such as an optical circulator or a directional coupler, a chirped fiber grating, and an optical amplifier that is optional. The directing unit has a first, second and third port. An optical signal is directed from the first port to the second port, where the chirped fiber grating is connected. The reflected light from the chirped fiber grating is directed from the second port to the third port, where the transmission optical path is connected. The chirped fiber grating gives the chromatic dispersion characteristic opposite to the chromatic dispersion characteristic of an output transmission path. In the optical transmitter, the electrical SCM signals pass through the pre-distortion circuit, and then externally/directly modulating an optical carrier via the optical modulator, which consists of a directly modulated semiconductor laser diode, or an external modulator with a CW laser diode. The modulated optical signal is launched to the directing unit. An optional optical amplifier can be installed right after the optical modulator or before the output transmission path, and can be a part of the optical transmitter.
The chirped fiber grating and output transmission path have opposite chromatic dispersion characteristic, which means that if the transmission path is a 1.3 xcexcm-band single mode fiber, the chirped fiber grating has negative chromatic dispersion in the wavelength range used. On the other hand, if the transmission path is a 1.55 xcexcm-band single mode fiber, the chirped fiber grating has positive chromatic dispersion in the wavelength range used.
According to the optical fiber transmitter of the present invention, a properly designed electrical pre-distortion circuit can be used to significantly reduce the effects of non-ideal chirped fiber grating, and the extra or insufficient chromatic dispersion in using the chirped fiber grating. Furthermore, by using an optical circulator, loss of insertion can be reduced. Alternatively, one can also use a directional coupler instead of an optical circulator.
According to the optical fiber transmitter of the present invention, long-distance SCM lightwave systems minimizing the limitations due to fiber chromatic dispersion and other nonlinearities can become feasible.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.